The potential of human ES cells has prompted national research agencies to stimulate the rapid development of this field. In the United States, NIH has awarded support to several Universities to stimulate the use of human ES cells. In addition, NIH has established a group at the Bethesda campus to acquire human ES cells from multiple sources and compare their properties. The lines currently available have been generated by groups using different techniques. The cells are hard to grow and the central function of the NIH unit is to define general strategies that allow these cells to be widely and confidently used in research. The NIH stem cell unit has 20 of the 22 federally approved lines and 8 of these have been cultured. Cells stocks have been established and these lines have been tested for viral contamination, karyotype integrity and for a panel of polymorphisms that identify the different cell lines. The initial stocks of cells were established using the different procedures initially used to establish the lines. The supplier?s protocols have been replaced with a universal protocol that may be used, with minor modification, to grow all cells. Embryonic stem cells readily differentiate and this makes the cells hard to sustain. Simple antibody based tests to define the undifferentiated state have been established. Cells are living organisms and they can become genetically altered. The ability to isolate new strains or sustain the integrity of the parent strain depends on growing new stocks derived from a single cell, cell clones. Procedures have been developed to achieve this. A website reports details of these protocols as well as other generally useful methods. Interactions with other NIH intramural labs have been established and the stem cell facility has also provided human ES cells to advanced courses attended by scientists from many countries.